The present invention relates to a control method for an electronic control thermostat which is used to control the temperature of cooling water in a cooling water temperature control system of an internal combustion engine (to be referred to as xe2x80x9cenginexe2x80x9d hereinafter) used in an automobile or the like in which the temperature of cooling water is set variably in accordance with the engine load.
A water-cooling type cooling device which uses a radiator is typically used in an automobile engine to cool the engine. A thermostat which uses a thermal expansion body to adjust the amount of cooling water recirculated on the radiator side or an electronically controlled valve unit has been used conventionally in this type of cooling device to control the temperature of the cooling water which is introduced into the engine with the aim of improving combustion efficiency in the automobile.
More specifically, a control valve of such a thermostat using a thermal expansion body or an electronically controlled valve unit is interposed in one portion of a cooling water passage such that when the temperature of the cooling water is low, the control valve is closed such that the cooling water is recirculated through a bypass passage rather than through the radiator, and when the temperature of the cooling water increases, the control valve is opened such that the cooling water is recirculated through the radiator. Thus the temperature of the cooling water can be controlled to a desired state.
Conventionally, the cooling water temperature control described above is performed by setting a target temperature arbitrarily. In some control systems employed in commercially available automobiles, for example, control is performed in accordance with a map comprising temperatures inputted in advance and data which is calculated in an engine control unit from various parameters such as the cooling water temperature, outside air temperature, vehicle speed, and engine rotation speed, for example, and linear control is hoped to be achieved by finely dividing these set temperatures.
It is also known that combustion efficiency in an automobile can be enhanced by reducing the cooling water temperature when the engine is running at high load and increasing the cooling water temperature when the engine is running at low load.
A large number of devices and methods employing various control systems have been disclosed in the background art with the object of improving combustion efficiency by performing cooling water temperature control to a desired state.
In Japanese Patent Application Laid-Open Publication No. H5-332136, for example, a method is disclosed as a method for controlling the temperature of cooling water in an engine in which precise temperature control is performed in accordance with the operating region such that even rapid rises in the temperature of the cooling water can be sufficiently addressed. In this conventional example, sensors for detecting temperature are provided in a cooling water passage on the inlet side and outlet side of the engine, and the detected values thereof are used selectively in accordance with the engine load to control the opening and closing of a control valve.
A cooling control device and cooling control method for an internal combustion engine (engine) are disclosed in Japanese Patent Application Laid-Open Publication No. H10-331637 in which temperature change in the cooling water is reduced as far as possible under any state of operation such that the engine runs at as high a temperature as possible without overheating. This conventional example is programmed to perform so-called constant water temperature control in which appropriate parameters indicating the operating state of the engine are used to read temperature drops in the cooling water from a table-format map, whereby temperature management is performed by predicting changes in the temperature of the cooling water.
In Japanese Patent Application Laid-Open Publication No. H5-222932, a cooling control device for an internal combustion engine (engine) is disclosed in which signals from a pressure sensor and an intake air temperature sensor which detect the density of intake air are read to calculate the density of the intake air, and control is performed such that the temperature on the inlet side of the engine is reduced further as the density increases and the temperature of the cooling water is increased further as the density decreases. In this conventional example, so-called map control is performed to control the cooling water temperature by detecting engine operating conditions such as the engine rotation speed and load and reading set temperatures from a map which is preset on the basis of these operating conditions.
Such devices and methods which perform cooling water temperature control through constant water temperature control or map control as described above have been disclosed in the background art in large numbers, but each has drawbacks and advantages and there is yet to be disclosed a device or method which is capable of further improving combustion efficiency by performing efficient cooling water temperature control in accordance with any operating state.
For example, the cooling water temperature control of the aforementioned conventional examples has defects such as the following.
That is, if cooling water temperature control is performed with great precision, the volume of data increases such that further labor is required and costs rise.
Further, it is impossible in actuality to control the temperature of cooling water synchronously with the required state of the engine. This is because the engine state changes constantly, and hence even if an attempt is made to calculate this state using a central processing unit CPU such as an engine control unit ECU and transmit signals to a thermostat valve or the like in order to alter the cooling water temperature to a target water temperature, a delay caused in the meantime by water temperature hunting or the like is inevitable. In short, even when such control is performed, several seconds or more are always required to reach the target temperature.
It is said that combustion efficiency may be improved during normal operations and mode operations by correcting various effective cooling water temperatures, ignition timings, and so on to optimum values using the engine control unit ECU. However, this is dependent upon certain fixed conditions being satisfied, and the actual effect thereof when a regular driver, and particularly a novice driver, drives normally is often small.
Furthermore, achieving linear control of the cooling water temperature to an optimum water temperature in conjunction with the actual state of operation is difficult from the point of view of responsiveness due to the course which must be followed to achieve the target temperature, namely detection of the cooling water temperature, calculation and control in the engine control unit ECU, operation of the electronic control thermostat on the basis thereof, and alteration of the flow of the cooling water in accordance therewith.
The present invention has been designed in consideration of such circumstances, and it is an object thereof to provide a control method for an electronic control thermostat which appropriately predicts and determines load variation in an engine in an operative state to thereby control the temperature of cooling water appropriately and efficiently such that an improvement in combustion efficiency can be achieved with a greater degree of reliability and in substantially all regions of the operative state.
In order to achieve such an object, a control method for an electronic control thermostat according to the present invention is a control method in an automobile engine cooling water temperature control system in which the temperature of cooling water is set variably by an electronic control thermostat in accordance with the engine load, this method being characterized in that parameters from a variety of sensor types which detect the state of the engine are inputted into an engine control unit, and when the engine control unit determines from the values of the parameters indicating the operative state of the automobile that the engine load is about to decrease, control is switched in the electronic control thermostat to a control method (so-called high temperature constant water temperature control) in which a target temperature for controlling the cooling water temperature is maintained at a fixed high temperature at all times, and when the engine control unit determines that medium or high loads are due to increase, control is switched in the electronic control thermostat to a control method (so-called map control) in which a target temperature corresponding to the parameter values is read from the engine control unit.
In this case, the control method for an electronic control thermostat according to the present invention is characterized in that when the accelerator opening, engine rotation speed, and so on, which are parameters indicating the operative state of the automobile, satisfy predetermined conditions, the engine control unit determines by prediction whether the engine load is low load, medium load, or high load.
The control method for an electronic control thermostat according to the present invention is also characterized in that the control (so-called high temperature constant water temperature control) performed by the engine control unit wherein a target water temperature for controlling the cooling water temperature is maintained at a high temperature at all times in the electronic control thermostat is performed on the basis of the throttle opening, the engine rotation speed, and the cooling water temperature.
The control method for an electronic control thermostat according to the present invention is further characterized in that the control (so-called map control) of the electronic control thermostat which is performed by reading a target temperature corresponding to the parameter values from the engine control unit is performed on the basis of at least one of or any combination of the throttle opening, the engine rotation speed, the cooling water temperature, the atmospheric pressure, the intake air amount, the intake air humidity, and the intake air temperature.
The control method for an electronic control thermostat according to the present invention is further characterized in that the electronic control thermostat is capable of variably controlling the cooling water temperature to any temperature, and is disposed in an engine cooling water passage on either the inlet side or the outlet side of the engine.
According to the present invention, when the cooling water temperature is controlled in accordance with the engine load (to a high water temperature in a low load and to a low water temperature in a medium or high load) with the object of enhancing combustion efficiency, for example, the engine load is predicted and determined to determine whether the automobile is in high load mode or low load mode.
As examples thereof, the automobile is usually in low load mode following engine start-up. Further, it is determined that the automobile is in high load mode when a high speed is maintained for at least a fixed time period or the throttle opening increases frequently. When manual mode or sport mode is entered in an automatic car, high load mode is determined. In a vehicle equipped with a car navigation system, high load mode is determined on the basis of information from the car navigation system or the like that the car is traveling on an express highway or a mountain road.
When high load mode is switched to, map control is performed by reading [parameter values] from the engine control unit in order to realize a target water temperature in accordance with the load. Since the cooling water temperature falls faster than it rises, a low water temperature which is optimum for combustion is realized with high probability at the time of a high load.
During low load mode, constant water temperature control to a high water temperature (for example 110xc2x0 C.) is performed in order to maintain the cooling water temperature at a high temperature. In so doing, a high water temperature which is optimum for combustion at the time of a low load can be realized in the majority of the operative state.
Here, any electronic control thermostat which is capable of controlling water temperature arbitrarily may be used, for example a thermostat having a WAX-PTC type constitution in which the relationship to cooling water temperature is eliminated by combining an exothermic body such as PTC with a thermostat which uses wax, a butterfly type constitution, or a rotary valve constitution.